Toxic and hazardous wastes encompass a wide variety of chemical pollutants examples of which include distilled petroleum products, crude oil, substituted and unsubstituted aromatic and aliphatic organic solvents, chemical industry by products, PCB's, medical waste and waste products from the food industries such as fats and oils.
Landfills, illegal dumping and leakage from underground storage tanks has resulted in significant contamination of soil with these wastes. Efforts are being made at both the federal and state level to identify those sites requiring immediate clean up. Remediation generally includes excavating the contaminated soils and hauling them to an off-site facility for combustion or incineration. Incineration has been found to be not entirely satisfactory since the plants are often large, must be constructed in rural areas away from the population and often create pollution themselves by converting the solid and liquid toxic waste into air borne pollutants. In addition, the energy requirements of conventional prior art incineration is substantial, resulting in high operational costs.
In the past, the use of combustion or thermal remediation for the disposal of petroleum distillates and organic hazardous wastes has led to highly specific systems tailored to the particular pollutant to be treated. These prior art processes require the incorporation of extensive secondary pollution control systems to deal with the undesirable by products of incineration such as carbon monoxide, oxides of nitrogen, oxides of sulphur, ozone, particulates and heat emitted to the environment, otherwise known as thermal pollution. These control systems result in an overall increase in both the size and cost of the equipment. Such plants are not truly mobile and are constructed at a relatively high cost while at the same time being limited in use to locations requiring extended remediation efforts. Afterwards, the plant must be dismantled and removed from the site. Further, for treatment at toxic waste sites located in densely populated areas the mobile site remediation equipment must conform to more strict thermal and noise abatement requirements.
The preferred prior art method of hazardous waste incineration is indirect pyrolysis since that method produces the least amount of pollution by-products. However, indirect pyrolysis must be fueled by external sources and is therefore unattractive for use in incineration systems, particularly the self-contained mobile system.
Most prior art thermal processing of hazardous waste employs rotating drums, rotating kilns, liquid injection incinerators or fluidized bed technology. Fluidized bed systems are described in U.S. Pat. No. 5,186,901 (Bayer et al.), U.S. Pat. No. 5,101,714 (Hirschberg et al.) and U.S. Pat. No. 5,145,826 (Hirschberg et al.). Exemplary of rotary kiln systems are U.S. Pat. No. 5,176,445 (Mize), U.S. Pat. No. 5,152,233 (Spisak) and U.S. Pat. No. 4,974,528 (Barcell). Rotary drum type evaporators for use in the treatment of soils or other contaminated solid substrates are U.S. Pat. No. 5,164,158 (Brashears et al.), U.S. Pat. No. 5,170,726 (Brashears et al.) and U.S. Pat. No. 5,054,931 (Farnham et al.).
Prior art fluidized bed systems have been found ineffective for use with pyrolysis due to both the need for highly classified particulates and to char build-up. Also, it is extremely difficult and expensive to effectively seal a rotating cylinder such as a kiln or drum against the leakage of gases into or out of the reactor.
Incineration of solids is known to produce potentially valuable heat energy along with carbon dioxide, nitrogen and residual ash. Pyrolysis i.e. incineration with a deficiency of air, allows the recovery of certain fuel gases and chemicals generated from the waste. Thermal oxidation is a type of incineration employed for the oxidation of contaminated fumes generated during incineration of a solid. Although prior art systems have incorporated these various remediation techniques in an effort to maximize efficiency of waste destruction while minimizing fuel consumption, such attempts have been met with limited success.
The failure of the prior art to efficiently recycle energy is partly due to the inability of such systems to distinguish between different recycle streams. If recycle streams are used in a staged chemical reaction process of the type having numerous different chemical reactions taking place, the recycle streams themselves must have appropriate chemical and physical properties. For example, the reaction conditions required for pyrolysis are substantially different than those for an oxidative combustion process. If maximum combustion efficiency and energy recovery is to be obtained, recycle streams must be tailored to the type of combustion reaction occurring.
Most of the prior art processes for destruction of hazardous or toxic organic wastes are incinerative in nature. The process described herein however, differs in that it is a tightly controlled, staged thermochemical reaction process. Incineration on the other hand, is a relatively uncontrolled process employing a flame to oxidize combustible materials thereby yielding a wide, unpredictable variety of gas products and incombustible ash residues. Also both the combustive materials and residues remain resident in the process for the entire incineration cycle time. Conversely, thermochemical reaction processes are tightly controlled, and therefore consistently generate specific quantities of desired products and by-products.
The basic thermochemical reactions used in the process described herein are tailored cases of pyrolysis and combustion based upon thermochemical equilibria. Pyrolysis is an anaerobic (oxygen free) chemical reaction process which employs heat to reduce complex chemicals into their essential building blocks. In the case of hydrocarbons, the produced product gases, primarily methane and hydrogen, are suitable for use as a low value fuel gas. Pyrolysis also produces a solid char residue of finely divided carbon. Although the product concentrations may vary, pyrolysis will consistently produce the same products and by-products, independent of the identity of the initial hydrocarbons processed. Combustion is a controlled chemical reaction as well, and in the case of oxidative combustion, also produces heat and/or light. A controlled combustion process, using a fuel having a specific chemical composition, will produce consistent quantities and identities of desired reaction products and by-products.
In the process described herein, a tailored pyrolysis reaction using a calculated water vapor content, is employed to simultaneously (1) convert the hydrocarbon feedstock into gases having a desired fuel value and (2) prevent any solid carbon from forming by converting it to gases with a fuel value. These gases, are then used as fuel for the secondary oxidative combustion stages, the heat from which is recycled back to the endothermic evaporator/pyrolysis stage. Because the process is designed to operate under conditions (e.g. environmental clean-up) where the composition of hydrocarbons in the feed stock will be variable, post-processing of the exhaust gases from the pyrolyzer reactor is necessary to produce a high quality fuel gas for secondary oxidative combustion thermochemical processes. This fuel gas will therefore have a consistent composition, ensuring a tightly controlled and predictable thermochemical reaction processes will take place once it is combusted downstream.
To efficiently recycle the energy produced by the oxidative combustion process, two recycle streams are required having different chemical and physical properties. A first recycle stream is injected back into the flash evaporator section of the pyrolysis reactor. This stream is obtained from a first stage oxidative combustion process. It must be of high temperature and pressure and be substantially oxygen free. To obtain this recycle stream, the first stage of the oxidative combustion process is performed under oxygen poor conditions. A second recycle stream is also required to indirectly heat the evaporator pyrolyzer reactor through heat exchange. This recycle stream must near ambient pressure and be rich in both steam and oxygen. In the present invention, this recycle stream is provided by a second oxidative combustion stage, the hydrolyzer reactor/module, operated under appropriate conditions sufficient to produce a recycle stream having the desired properties.
No previous prior art system produces a high temperature, low oxygen, recycle gas stream for use as a direct carrier gas in the pyrolysis reaction. The present invention employs post-pyrolysis multistage combustion of the generated fuel gases. These gases are adjusted to the required sub-stoichiometric oxygen concentration followed by parallel combustion in either a thermal oxidizer reactor or a fuel gas combusting turbine. The resultant low oxygen, high temperature, high pressure exhaust gases from the turbine are directly recycled into the evaporator section of the pyrolysis chamber to assist in flash evaporation of the volatile contaminants from the solid material. The exhaust gases from the oxidizer reactor are in turn converted into blends of hot gases and steam for indirect heat exchange of the pyrolysis reactor.
A need has therefore existed in the art to provide a multi-stage system and method for the destruction of toxic and/or hazardous waste materials, via incineration and pyrolysis, which efficiently recycles the gaseous by-products generated from the destruction of the hazardous waste including heat recovery.